[{name:"t1"},{name:"t2"},{name:"t4"},{name:"t3"},{name:"t5"},{name:"t8"},{name:"t3"},{name:"t2"},{name:"t8"},]

db.collection.aggregate([{$group:{_id:null,data:{$push:"$$ROOT"}}},{$unwind:{path:"$data",includeArrayIndex:"counter",}},{$replaceRoot:{newRoot:{$mergeObjects:["$data",{counter:{$add:["$counter",1]}}]}}}])